Emulsions comprised of a dispersing medium and silver halide microcrystals or grains have found extensive use in photography. Radiation sensitive silver halide emulsions have been employed for latent image formation. The radiation sensitive silver halide grains employed in photographic emulsions are typically comprised of silver chloride, silver bromide, or silver in combination with both chloride and bromide ions, each often incorporating minor amounts of iodide. Radiation sensitive silver iodide emulsions, though infrequently employed in photography, are known in the art. Silver halide emulsions are known to be useful in photographic elements for purposes other than latent image formation, such as for radiation absorption or scattering, interimage effects, and development effects.
In general silver halides exhibit limited absorption within the visible spectrum. Progressively greater blue light absorptions are observed in silver chloride, silver bromide, and silver iodide. However, even silver iodide emulsions appear pale yellow, with their principal light absorption occurring near 400 nm.
The crystal structure of silver iodide has been studied by crystallographers, particularly by those interested in photography. The most commonly encountered crystalline class of silver iodide is the hexagonal wurtzite class, hereinafter designated .beta. phase silver iodide. Silver iodide of the face centered cubic crystalline class, hereinafter designated .gamma. phase silver iodide, is also stable at room temperature. The .beta. phase of silver iodide is the more stable of the two phases so that emulsions containing .gamma. phase silver iodide grains also contain at least a minor proportion of .beta. phase silver iodide grains.
Byerley and Hirsch, "Dispersions of Metastable High Temperature Cubic Silver Iodide", Journal of Photographic Science, Vol. 18, 1970, pp. 53-59, have reported emulsions containing a third crystalline class of silver iodide, the body centered cubic class, hereinafter designated .alpha. phase silver iodide. .alpha. phase silver iodide is bright yellow, indicating that it exhibits increased absorption in the blue portion of the spectrum as compared to .beta. and .gamma. phase silver iodide, which are cream colored. The emulsions containing .alpha. phase silver iodide studied by Byerley and Hirsch were unstable in that they entirely reverted to cream colored silver iodide at temperatures below 27.degree. C.
The teachings of Byerley and Hirsch are considered to represent the prior art most relevant to this invention. Additional art relating to silver iodide is identified and discussed in the Related Art Appendix following the Examples.